Many medical devices comprising polymers, such as diagnostic and balloon catheters, are currently being manufactured utilizing conventional thermoplastic polymer thermoforming techniques such as extrusion, injection molding, stretch blow molding, and the like. Within these processes, one softens or melts the polymer and reshapes it into the desired shape. Although these thermoforming processes are well developed, pressures exist to shrink the size of such medical products. At the same time the diversity of local functional properties within the device is increasing. Consequently, an increasing number of complex processing steps have to be taken to get to the desired result.
Balloon molding from thermoplastic polymer compositions comprising reinforcements is difficult due to the fact that most types of reinforcing agents are unlikely to deform during the blow molding process. Dip molding balloons is possible, but due to the fact the inner shape has to be removed from within the balloon, this is not the most suitable way to produce a reinforced balloon.
High tensile strengths are important in angioplasty balloons because they allow for the use of high pressure in a balloon having a relatively small wall thickness. High pressure is often needed to treat some forms of stenosis. Small wall thicknesses enable the deflated balloon to remain narrow, making it easier to advance the balloon through the arterial system. Similar factors are important in catheter shaft materials.
One of the disadvantages of blow molding balloons is that the cone sections have a thicker wall than the central section. This results in a large balloon profile during folding. A variety of techniques have been offered to reduce cone thickness, but they are not always suitable for a given balloon.
Because of these factors, fabrication techniques for such device components are not adequate to keep reducing size, increasing device complexity, and/or implementing new devices. Consequently there is a need for new fabrication techniques to provide a wider range of local functional properties at the same time allow further size reductions.
Curable compositions, dispensed or applied in liquid form, and subsequently cured have some uses in conventional fabrication of catheter devices, typically in adhesive or coating applications. However, prior to the inventions described herein they have not obtained widespread use.
Devices formed of cured polyimide materials have been described in several documents. Polyimide polymers, known for their high strength at very high temperatures are typically formed by heating polyamide-acid precursor polymer material to a curing temperature where amide and acid groups along the polymer condense to form cyclic imide groups in the backbone polymer chain. This technique is used to form balloons in Euteneuer, U.S. Pat. No. 4,952,357. This fabrication method, however, is unsuited to many device forming applications because of the high temperatures required for curing the polyimide. Further, while polyimide has excellent strength properties, the resulting polymers have relatively poor flexibility, elongation and softness. Still further, the manufacturing procedure uses HF to dissolve a glass substrate upon which the polyamide-acid is formed by deposition from solution. The glass substrate formation and subsequent HF destruction thereof is a relatively dangerous and expensive process. Polyimide tubing used for catheter shafts is described in U.S. Pat. No. 4,976,720, Machold et al, but with no discussion of how it is made.
U.S. Pat. No. 5,100,381, Burns, describes angioplasty catheters with shaft portions made of polyimide or polyimide-polytetrafluoroethylene composite material.
U.S. Pat. No. 6,024,722, Rau et al, applies thermoplastic polyimide to the art of balloon catheter construction, i.e., to catheter shafts, guide catheters, infusion catheters and balloons. Use of this material, however, is subject to the same limitations already recognized for the general class of thermoplastic polymers.
U.S. Pat. No. 5,145,942, Hergenrother, et al, describes methyl-substituted polyimide polymers which are thermoplastic, but curable to a crosslinked state by irradiation with UV or exposure to temperatures in excess of 275° C. The UV irradiation process, however, appears to be very slow (100 hrs at 0.21 watts/cm2 to cure films of 1.7 and 2.4 mils (0.04-0.06 mm). Thus use of this material for forming medical devices appears to offer few or no benefits compared to other polyimides, while at the same time incurring further processing disadvantages.
In addition to condensation from polyamide-acid polymers, it has been proposed to form a polyimide from a bis-maleimide compound by Diels Alder cycloaddition, however these reactions are also run at temperatures in excess of 200° C. More recently it has been proposed to prepare polyimides by diene cycloadditions which are catalyzed by UV irradiation, near or even below ambient temperatures.